The Western Bark Beetle Research
Group: A Unique Collaboration With
Forest Health Protection
Proceedings of a Symposium at the 2007
Society of American Foresters Conference
October 23–28, 2007, Portland, Oregon
J.L. Hayes and J.E. Lundquist, compilers
PA C I F I C N O R T H W E S T
RESEARCH STATION
U.S. Department of Agriculture, Forest Service
Pacific Northwest Research Station
Portland, Oregon
General Technical Report PNW-GTR-784
April 2009
Bark Beetles in a Changing Climate1
John E. Lundquist and Barbara J. Bentz2
Abstract
Over the past decade, native bark beetles (Coleoptera: Curculionidae) have killed
billions of trees across millions of hectares of forest from Alaska to Mexico. Although
bark beetle infestations are a regular force of natural change in forested ecosystems,
several current outbreaks occurring simultaneously across western North America are
the largest and most severe in recorded history. Bark beetle ecology is complex and
dynamic, and a variety of circumstances must coincide for a large scale bark beetle
outbreak. While outbreak dynamics vary from bark beetle species to bark beetle species
and from forest type to forest type, a combination of several factors appear to be driving
current outbreaks, including a changing climate.
Keywords: Global warming, climate change, latitudinal gradient, climate models, climate
normals.
___________________
1
The genesis of this manuscript was a presentation by the authors at the Western Bark Beetle Research
Group—A Unique Collaboration with Forest Health Protection Symposium, Society of American Foresters
Conference, 23–28 October 2007, Portland, OR.
2
John E. Lundquist is an Entomologist, USDA Forest Service, R-10 Forest Health Protection and Pacific
Northwest Research Station, Anchorage, AK; email: jlundquist@fs.fed.us. Barbara J. Bentz is a
Research Entomologist, USDA Forest Service, Rocky Mountain Research Station, Logan, UT; email:
bbentz@fs.fed.us.
39
Introduction
Over the past 100 years, global average temperature has risen by 0.74°C (0.56–0.92°C
range). The greatest increase has occurred during the last two decades and experts say
increases will continue (CIRMOUNT 2006). Predictions for increasing average global
temperatures range from 1.0 °C to 4 ºC over the next 100 years (Houghton et al. 2001).
There has also been a dramatic increase in the number of publications on climate
change. An internet search of “climate change” finds over 60,000 hits in 2007 alone!
Climate change may be one of the most focused topic areas in living history.
The amazing interest in this topic is not easy to explain. Most would agree that the
biological understanding and science underlying the climate change phenomenon has
existed for at least a couple of decades (Houghton et al. 2001). But science alone has
been inadequate in evoking such a response. Politics and the media apparently lined up
just right with science causing climate change to emerge from “science” to a truly
popular phenomenon (Boykoff 2007).
What is climate change?
“Climate” refers to the average state of the weather. Common weather phenomena are
temperature, rain, snow, fog, wind, cloud, dust storms, and events such as tornadoes,
hurricanes and ice storms. Weather usually refers to activity of these phenomena over
short periods of time (hours or days) in localized areas. Climate refers to average
atmospheric conditions over longer periods of time and involves broad areas. Although
climate change is usually portrayed as increasing temperature, it actually expresses
itself in many other ways as well; e.g., as changes in precipitation, UV-B radiation,
atmospheric CO2, nitrogen deposition, and others.
We see effects of climate change easier than we can experience a changing climate.
These changes impact many things (Kolbert 2006, Parmesan 2006, Roy and Sparks
2000, Wohlforth 2002). We have heard about glaciers melting, flowers blooming earlier
than in previous years, ocean levels rising and ocean-front villages washing to the sea,
butterfly distributions migrating north (and one or two going south), and bark beetle
outbreaks killing millions of hectares of forests. Climate is one of those unique
phenomena that has the ability to effect nearly everything. It has been referred to as the
“ultimate integrative field”.
Why should we be interested in the effects of climate on herbivorous insects,
like bark beetles?
Insects have short life cycles, resulting in dozens or hundreds of generations in the time
it takes most higher plants to complete one generation. Because insect life cycles are
environmentally driven, a change in climate can significantly influence insect population
timing and density. Monitoring insect population trends can then be used as an indirect
measure of climate change. Furthermore, insects occur nearly everywhere and many
can be studied year round. Insect pests can shape or change ecosystem structure and
function, and, in doing so, act as catalysts of change. Insects can help maintain or
40
sustain ecosystems; displace or remove components of ecosystems; or lead to
replacement of existing ecosystems. Insects can respond to long-term subtle shifts in
their environment that are commonly so subtle that they cannot be directly experienced
by humans. In short, insects can serve as a convenient bioindicator of climate change.
Bark beetles, in particular, create the most visible of insect disturbances in a forest
because they kill trees, lots of trees, and their impacts vary across all ecosystem
services provided by a forest (Fettig et al. 2007). They influence nutrient cycling, energy
flow, decomposition and other supporting services of ecosystems. They affect wood
production and other provisioning services and goods of ecosystems. They influence
water production, snow distribution and other regulating functions of ecosystems. They
impact recreational experiences and other cultural services. They react quickly to
changes in climate; much faster than higher plants including trees. Many bark beetle
species have geographic distributions less extensive than their tree hosts (Ayres and
Lombardero 2000), which suggests distributions could rapidly shift with climate change
(Carroll et al. 2004).
Climate change can directly affect bark beetle phenology and winter mortality, resulting
in shifts in length and number of annual life cycles. Bark beetle communities will also be
affected including predator/prey relationships, interactions with symbiotic fungi, forest
structure, and forest vigor. Changes in temperature, precipitation and atmospheric
gases will undoubtedly affect host tree defenses as well, possibly resulting in changes
to bark beetle host specificity and geographic distribution. Rapid changes in climate
may also result in genetic adaptations that create metapopulations (Balanya et al. 2006,
Bradshaw and Holzapfel 2006). Spatial and temporal synchrony of beetles and their
host trees may also be disrupted.
Dramatic increase in outbreaks of bark beetles in the West
Western U.S. states and Canadian provinces have recently seen a significant increase
in bark beetle activity. Examples include pinyon ips (Ips confusus LeConte) on pinyon
pine (mostly Pinus monophylla Torr. & Frem. and P. edulis Engelm.) in the
southwestern U.S. (Breshears et al. 2005), spruce beetle (Dendroctonus rufipennis
Kirby) in Alaska (Werner et al. 2006), mountain pine beetle (D. ponderosae Hopkins)
along the Rocky Mountain Front Range in Colorado (Negrón and Popp 2004), in high
elevation forests (Gibson 2006, Bentz and Schen-Langenheim 2007), and in interior
British Columbia (Westfall and Ebata 2008). Climate change has been implicated as a
major influencing factor (Berg et al. 2006, Breshears et al. 2005, Nijhuis 2004). Proving
a direct correlation between climate and bark beetle outbreaks, however, is a difficult
task.
Understanding how climate affects the mechanics of bark beetle outbreaks is a
challenge
Climate affects everything in an already complicated biological system. For a bark
beetle outbreak to occur, there must be suitable climate for several years, an active
beetle population, and an extensive area of host trees of appropriate age, size and
species (Fettig et al. 2007). Temperature, moisture and other climatic elements
41
symbolic of a changing climate can affect these requirements for a bark beetle
outbreak. Outbreaks are often non-linear, unpredictable, sometimes unexpected events
(Logan et al. 2003). Bark beetle outbreaks result from a unique combination of
conditions at a variety of scales (Raffa et al. 2008).
Elevated temperature and shifting precipitation patterns, in particular, appear to be
influencing recent and current bark beetle outbreaks (Régnière and Bentz 2007, Shaw
et al. 2005). Elevated temperatures can speed up reproductive and growth cycles and
reduce cold-induced mortality during cold snaps (Bentz and Mullins 1999, Bentz et al.
1991, Logan and Bentz 1999). Although the relationship is nonlinear, prolonged drought
can weaken trees, making them more susceptible to bark beetle attacks (Breshears et
al. 2005, Mattson and Haack 1987, Waring and Cobb 1992).
Because bark beetles are one component of a rich community comprising forest
ecosystems, to fully understand climate change effects on bark beetles and hence
forest ecosystems, we need to consider how climate change influences biotic
interactions of symbiosis, competition, predation and other dynamic disturbance
processes (Botkin et al. 2007). For example, Six and Bentz (2007) observed that
temperature determines the relative presence of symbiotic fungi associated with
mountain pine beetle. Although relationships are unclear at this time, it is obvious that
climate change effects on fungal populations will have a cascading effect on mountain
pine beetle population success. Effects of climate change on other critical components
of bark beetle communities, including predators and parasites, are also unclear.
Predicting climate and weather events of the future is a very difficult task
Forecasts are less reliable the further out in time they project. Small, seemingly
insignificant, changes can amplify to become major system shifts, which are
unpredictable (Burkett et al. 2005). Because of a sensitivity to small changes, it will
never be possible to make perfect forecasts (Holling 2001), although there still is much
potential for improvement. Useful predictions of future insect activity will depend on
reliable predictions of weather and a good understanding of cause/effect relations
between weather/climate and insect physiology, behavior, and ecology (Stireman et al.
2005). Predicting the future involves some very complex mathematical models and very
advanced, high capacity computers (McKenney et al. 2003, Rehfeldt et al. 2006,
Williams and Liebhold 2002).
Mechanistic mathematical models have been developed to describe and predict
mountain pine beetle phenology (Bentz et al. 1991, Gilbert et al. 2004, Jenkins et al.
2001, Logan and Bentz 1999, Powell et al. 2000, Safranyik et al. 1975) and cold
tolerance (Régnière and Bentz 2007), and spruce beetle voltinism (Hansen et al.
2001b). These models have been implemented within the BioSim (Régnière and StAmant 2007) modeling framework, enabling landscape-scale projections of population
success given daily temperatures for the duration of a generation, one, two or three
years depending on bark beetle species and geographic location.
42
Model results using climate-changed normals suggest that the probability of mountain
pine beetle temperature-dependent survival in western U. S. over the next 25 yrs will
generally increase. High-elevation forests will experience the greatest increase in
probability of mountain pine beetle survival. The biggest increase in univoltine spruce
beetle populations, and thus exponential population growth, is predicted to occur in
Alaska and high-elevation areas of the western U.S. Historically, spruce beetle has had
a two-year, or in some cases three-year, life cycle in these areas.
Our ability to predict western U.S. bark beetle response to climate change is limited by a
lack of data on species-specific temperature-dependent developmental processes. As
described above, we do have models for mountain pine beetle and spruce beetle,
although additional research is needed to parameterize existing models to account for
regional genetic differences in population response to temperature. For other bark
beetle species, our current ability to forecast climate change effects on population
dynamics is almost entirely qualitative.
What can we expect to happen to bark beetle populations as climate changes?
Many possible scenarios have been proposed. In general, insect outbreaks are
probably going to increase in number and severity (Ayres and Lombardero 2000,
Stireman et al. 2005). Interactions between insects and their natural control agents
(parasites, pathogens, predators, parasitoids) may be disrupted resulting in positive or
negative effects on insect populations (Malmstrom and Raffa 2000). Host plant and
insect phenological synchrony may be disrupted. Winter survival of insects may
increase (Regniere and Bentz 2007, Williams and Liebhold 2002). Observed genetic
adaptation to local environmental conditions (Bentz et al. 2001) suggest that bark
beetles could rapidly respond to a changing climate. Exotic insects will have more
opportunities to invade new areas and previously innocuous insects may shift hosts and
become pests (Pernek et al. 2008). Distributions will shift northward in latitude and
upward in elevation. Bark beetle species currently restricted to the southern U.S. and
Mexico could expand northward. Northernmost forests will be affected first and most
severely (Thomas et al. 2006).
Bark beetle management under a changing climate
Managers want to know what can be done to hedge against future effects under such a
cloud of uncertainty. Several management coping strategies have been proposed
including a change in forest structure and age patterns across landscapes, altered
species composition and diversity, reduction in invasive species populations, prompt
action when new invasives are detected, planting late successional species, and many
others (Spittlehouse and Stewart 2003). Most of these suggestions are based on logic
alone, since unprecedented conditions are facing managers. Few are based on
statistically rigorous experimental research. Managers must be willing to accept that
climate change will result in novel environmental conditions never experienced by
current forest ecosystems, and dynamic strategies that enhance ecosystem adaptability
will be required (Millar et al. 2007).
43
Conclusions
Management traditionally has been aimed at recreating the past using such concepts as
historical range of variability (Choi 2007). We treat the past as a stable state. We are
beginning to realize now that we have no such stable state under a changing climate.
We are tremendously challenged to predict what future suitably resilient environments
will look like. The future is a moving target. One thing that is highly probable… the
climate will change. Which way it changes is a question on many researchers and
practitioners minds. Extrapolating the climate versus time curve is a challenging effort,
and some believe that taking actions in response to climate change can create a bigger
risk than doing nothing (Spittlehouse and Stewart 2003).
There are many unanswered questions about potential effects of climate change on
western bark beetle populations. Many will be difficult, perhaps impossible, to answer.
Management of western U.S. forest ecosystems should be based on the best available
science, a prospect facilitated by scientists within U.S. Forest Service Research and
Development Western Bark Beetle Research Group.
Acknowledgements
We thank Ken Gibson, Mark Schultz, Chris Fettig, Aileen Holthaus, and Steve Patterson
for reviewing earlier versions of this manuscript. This paper was originally presented at
the 2007 Annual Meeting of the Society of American Foresters. We are grateful to SAF.
Literature Cited
Ayres, M.P.; Lombardero, M.J. 2000. Assessing the consequences of global change
for forest disturbance from herbivores and pathogens. The Science of the Total
Environment. 262: 263–286.
Balanya, J.; Oller, J.M.; Huey, R.B.; Gilchrist, G.W.; Serra, L. 2006. Global
genetic tracks global climate warming in Drosophila subobscura. Science.
313: 1773–1775.
Bentz, B.J.; Mullins, D.E. 1999. Ecology of mountain pine beetle cold hardening in the
Intermountain West. Environmental Entomology. 28(4): 577–587.
Bentz, B.J.; Schen-Langenheim, G. 2007. The mountain pine beetle and whitebark
pine waltz: has the music changed? Proceedings of the Conference Whitebark
Pine: A Pacific Coast Perspective. http://www.fs.fed.us/r6/nr/fid/wbpine/papers/
2007-wbp-impacts-bentz.pdf.
Bentz, B.J.; Logan, J.A.; Amman, G.D. 1991. Temperature-dependent development of
the mountain pine beetle (Coleoptera: Scolytidae) and simulation of its
phenology. Canadian Entomologist. 123: 1083–1094.
44
Berg, E.E.; J.D. Henry; C.L.Fastie; A.D. De Volder; S.M. Matsuoka. 2006. Spruce
beetle outbreaks on the Kenai Peninsula, Alaska, and Kluane National Park
and Reserve, Yukon Territory: relationship to summer temperatures and
regional differences in disturbance regimes. Forest Ecology and Management.
227: 219–232.
Botkin, D.B.; Saxe, H.; Araújo, M.B.; Betts, R.; Bradshaw, R.H.W.; Cedhagen, T.;
Chesson, P.; Dawson, T.P.; Etterson, J.R.; Faith, D.P.; Ferrier, S.; Guisan,
A.; Hansen, A. S.; Hilbert, D.W.; Loehle, C.; Margules, C.; New, M.; Sobel,
M.J.; Stockwell, D.R.B. 2007. Forecasting the effects of global warming on
biodiversity. BioScience. 57: 227–236
Boykoff, M. 2007. Changing patterns in climate change reporting in United States and
United Kingdom media. Presentation at Carbonundrums: making sense of
climate change reporting around the world. Reuters Institute for the Study of
Journalism & Environmental Change. June 27, 2007. http://www.eci.ox.ac.uk/
news/events/070727-carbonundrum/boykoff.pdf. (12 January 2009).
Bradshaw, W.E.; Holzapfel, C.M. 2006. Evolutionary response to rapid climate
change. Science. 312: 1477–1478.
Breshears, D.D.; Cobb, N.S.; Rich, P.M.; Price, K.P.; Allen, C.D.; Balice, R.G.;
Romme, W.H.; Kastens, J.H.; Floyd, M.L.; Belnap, J.; Anderson, J.J.; Myers,
O.B.; Meyer, C.W. 2005. Regional vegetation die-off in response to globalchange-type drought. Proceedings of the National Academy of Science.
102: 15144–15148.
Burkett, V.R.; Wilcox, D.A.; Stottlemyer, R.; Barrow, W.; Fagre, D.; Baron, J.;
Price, J.; Nieldsen, J.L.; Allen, C.D.; Peterson, D.L.; Ruggerone, G.; Doyle,
T. 2005. Nonlinear dynamics in ecosystem response to climatic change: case
studies and policy implications. Ecological Complexity. 2: 357–394.
Carroll, A.L.; Taylor, S.W.; Regniere, J.; Safranyik, L. 2004. Effects of climate
change on range expansion by the mountain pine beetle in British Columbia. In:
Shore, T.L.; Brooks, J.E.; Stone, J.E., eds. Mountain pine beetle symposium:
challenges and solutions. October 30–31, 2003, Kelowna, British Columbia.
Information Report BC-X-399. Victoria, BC: Natural Resources Canada,
Canadian Forest Service, Pacific Forestry Centre. 298 p.
Choi, Y.D. 2007. Restoration ecology to the future: a call for new paradigm. Restoration
Ecology. 15(2): 351–353.
45
CIRMOUNT Committee. 2006. Mapping new terrain: climate change and America’s
West. Report of the Consortium for Integrated Climate Research in Western
Mountains (CIRMOUNT). Misc. Pub., PSW-MISC-77. Albany, CA: US
Department of Agriculture, Forest Service, Pacific Southwest Research
Station. 29 p.
Fettig, C.J.; Klepzig, K.D.; Billings, R.F.; Munson, A.S.; Nebeker, T.E.; Negrón,
J.F.; Nowak, J.T. 2007. The effectiveness of vegetation management practices
for prevention and control of bark beetle outbreaks in coniferous forests of
the western and southern United States. Forest Ecology and Management.
238: 24–53.
Gibson, K. 2006. Mountain pine beetle conditions in whitebark pine stands in the
greater Yellowstone ecosystem, 2006. Numbered Report 06-03. Missoula, MT:
U.S. Department of Agriculture, Forest Service, Forest Health Protection. 7 p.
Gilbert, E.; Powell, J.A.; Logan, J.A.; Bentz, B.J. 2004. Comparison of three models
predicting developmental milestones given environmental and individual
variation. Bulletin of Mathematical Biology. 66: 1821–1850.
Hansen, E.M.; Bentz, B.J.; Turner, D.L. 2001a. Physiological basis for flexible
voltinism in the spruce beetle (Coleoptera: Scolytidae). Canadian Entomologist.
133: 805–817.
Hansen, E.M.; Bentz, B.J.; Turner, D.L. 2001b. Temperature-based model for
predicting univoltine brood proportions in spruce beetle (Coleoptera: Scolytidae).
Canadian Entomologist. 133: 827–841.
Holling, C.S. 2001. Understanding the complexity of economic, ecological, and social
systems. Ecosystems. 4: 390–403.
Houghton, J.T.; Ding, Y.; Griggs, D.J.; Noguer, M.; van der Linden, P.J.; Dai, W.;
Maskell, K.; Johnson, C.A., eds. 2001. Climate change 2001: the scientific
basis. Contribution of Working Group I to the Third Assessment Report of the
Intergovernmental Panel on Climate Change. Cambridge University Press,
Cambridge and New York. 881 p.
Jenkins, J.L.; Powell, J.A.; Logan, J.A.; Bentz, B.J. 2001. Low seasonal
temperatures promote life cycle synchronization. Bulletin of Mathematical
Biology. 63: 573-595.
Kolbert, E. 2006. Field Notes from a Catastrophe: Man, Nature, and Climate Change.
Atlantic Monthly Press, N.Y. 357 p.
Logan, J.A.; Regniere, J.; Powell, J.A. 2003. Assessing the impact of global warming
on forest pest dynamics. Frontiers in Ecology and the Environment. 1(3): 130–137.
46
Logan, J.A.; Bentz, B.J. 1999. Model analysis of mountain pine beetle (Coleoptera:
Scolytidae) seasonality. Environmental Entomology. 28(6): 924–934.
Malmstrom, C.M.; Raffa, K. F. 2000. Biotic disturbance agents in the boreal
forest: considerations for vegetation change models. Global Change Biology.
6(1): 35–48.
Mattson, W.J.; Haack, R.A. 1987. The role of drought in outbreaks of plant-eating
insects. BioScience. 37: 110–118.
McKenney, D.W.; Hopkin, A.A.; Campbell, K.L.; Mackey, B.G.; Roottit, R. 2003.
Opportunities for improved risk assessments of exotic species in Canada using
bioclimatic modeling. Environmental Monitoring and Assessment. 88: 445–461.
Millar, C.I.; Stephenson, N.L.; Stephens, S.L. 2007. Climate change and forests
of the future: managing in the face of uncertainty. Ecological Applications.
17(8): 2145–2151.
Negrón, J.F.; Popp, J.B. 2004. Probability of ponderosa pine infestation by mountain
pine beetle in the Colorado Front Range. Forest Ecology and Management.
191: 17–27.
Nijhuis, M. 2004. Global warming’s unlikely harbingers. High Country News, July
19, 2004.
Parmesan C. 2006. Ecological and evolutionary responses to recent climate change.
Annual Review of Ecology, Evolution and Systematics. 37: 637–669.
Pernek, M.; Pilas, I.; Vrbek, B.; Benko, M.; Hrasovec, B.; Milkovic, J. 2008.
Forecasting the impact of the Gypsy moth on lowland hardwood forests by
analyzing the cyclical pattern of population and climate data series. Forest
Ecology and Management. 255: 1740-1748.
Powell, J.A.; Jenkins, J.L.; Logan, J.A.; Bentz, B.J. 2000. Seasonal temperature
alone can synchronize life cycles. Bulletin of Mathematical Biology. 62: 977–998.
Raffa, K.F; Aukema, B.H.; Bentz, B.J.; Carroll, A.L.; Hicke, J.A.; Turner, M.G.;
Romme, W.H. 2008. Cross-scale drivers of natural disturbances prone to
anthropogenic amplification: dynamics of biome-wide bark beetle eruptions.
BioScience. 58(6): 501–518.
Régnière, J.; Bentz, B.J. 2007. Modeling cold tolerance in the mountain pine beetle,
Dendroctonus ponderosae. Journal of Insect Physiology. 53: 559–572.
Régnière J.; St-Amant, R. 2007. Stochastic simulation of daily air temperature and
precipitation from monthly normals in North America north of Mexico.
International Journal of Biometeorology. 51: 415–430.
47
Rehfeldt, G.E.; Crookston, N.L.; Warwell, M.V.; Evans, J.S. 2006. Empirical analyses
of plant-climate relationships for the western United States. International Journal
of Plant Science. 167: 1123–1150.
Roy, D.B; Sparks, T.H. 2000. Phenology of British butterflies and climate change.
Global Change Biology. 6: 407–-416.
Safranyik L.; Shrimpton D.M.; Whitney H.S. 1975. .An interpretation of the
interactions between lodgepole pine, the mountain pine beetle, and its
associated blue stain fungi in western Canada. In: Baumgartner D.M., ed.
Management of lodgepole pine ecosystems. Pullman, WA: Washington State
University Cooperative Extension Service: 406–428.
Shaw, J.D.; Steed, B.E.; DeBlander, L.T. 2005. Forest inventory and analysis (FIA)
annual inventory answers the question: what is happening to pinyon-juniper
woodlands? Journal of Forestry. 103: 280–285.
Six, D.L.; Bentz, B.J. 2007. Temperature determines the relative abundance of
symbionts in a multipartite bark beetle-fungus symbiosis. Microbial Ecology.
54: 112–118.
Spittlehouse, D.L.; Stewart, R.B. 2003. Adaptation to climate change in
forest management. BC Journal of Ecosystems and Management. 4(1).
http://www.forrex.org/jem/2003/vol4/no1/art1.pdf.
Stireman, J.O. III; Dyer, L.A.; Janzen, D.H.; Singer, M.S.; Lill, J.T.; Marquis, R.J.;
Ricklefs, R.E.; Gentry, G.L.; Hallwachs, W.; Coley, P.D.; Barone, J.A.;
Greeney, H.F.; Connahs, H.; Barbosa, P.; Morais, H.C.; Diniz, I.R. 2005.
Climatic unpredictability and parasitism of caterpillars: implications of global
warming. Proceedings of the National Academy of Sciences. 102: 17384–17387.
Thomas, C.D.; Franco, A.M.A.; Hill, J.K. 2006. Range retractions and extinction in the
face of climate warming. Trends in Ecology and Evolution. 21(8): 415–416.
Waring, G.L.; Cobb, N.S. 1992. The impact of plant stress on herbivore population
dynamics. In: Bernays, E., ed. Insect-plant interactions, vol. IV. Boca Raton, FL:
CRC Press: 167–226.
Werner, R.A.; Holsten, E.H. 1985. Effect of phloem temperature on development of
spruce beetles in Alaska. In: Safrankik, L., ed. The role of the host in the
population dynamics of forest insects. Victoria, BC: Forest Canada, Pacific
Forest Centre: 155–163.
Werner, R.A.; Holsten, E.H.; Matsuoka, S.M.; Burnside, R.E. 2006. Spruce beetles
and forest ecosystems in south-central Alaska: a review of 30 years of research.
Forest Ecology and Management. 227: 195–206.
48
Westfall, J.; Ebata, T. 2008. Summary of forest health conditions in British Columbia 2007. Pest Management Report No. 15. Victoria, BC: British Columbia Ministry of
Forests, Forest Practices Branch. 81 p. http://www.llbc.leg.bc.ca/public/PubDocs/
bcdocs/354419/bc_forests_hlth_con_2007.pdf.
Williams, D.W.; Liebhold, A.M. 2002. Climate change and the outbreak ranges of two
North American bark beetles. Agricultural and Forest Entomology. 4: 87–99.
Wohlforth, C. 2002. Spruce bark beetles and climate change. Alaska Magazine.
March 2002. http://www.wohlforth.net/FullTextArticles.html (6 January 2009)
49